EP0061552B1

EP0061552B1 - Initially erasable ink composition for a ball point writing instrument

Info

Publication number: EP0061552B1
Application number: EP81306011A
Authority: EP
Inventors: Frank Andrew Muller
Original assignee: Scripto Tokai Inc; Scripto Inc
Current assignee: Scripto Tokai Inc
Priority date: 1981-03-25
Filing date: 1981-12-21
Publication date: 1986-05-07
Also published as: GB2095270A; KR830009177A; CA1183983A; JPS57170967A; BR8201623A; ZA822032B; EP0061552A1; DK132282A; ZW5382A1; NZ200112A; KR890003443B1; ATE19644T1; GR76460B; MX160264A; PH16936A; AU548090B2; TR21219A; DE3174593D1; GB2095270B; US4329262A

Abstract

An initially erasable ink composition for a ball point writing instrument which ink is characterized by its initial erasability by ordinary pencil erasers when applied by a ball point writing instrument to an absorbent paper-like writing surface and which thereafter develops permanence. The ink composition comprises an elastomer selected from the group consisting of natural rubbers, synthetic rubbers and mixtures thereof, pigment, and a solvent system for said elastomer and pigment. The elastomer is present in the range of about 18-28% by weight of the ink and the pigment is present in the range of about 10-22% by weight of the ink. The solvent system comprises a volatile component present in the range of about 8-30% by weight of the ink and having a boiling point less than about 180 DEG C. and an essentially non-volatile component which is predominantly a low viscosity hydrocarbon oil, essential oil, petroleum derivative, plasticizer or mixtures thereof having a boiling point less than about 300 DEG C. and greater than about 180 DEG C. being present in the range of about 21 to 50.5% by weight of the ink. The solvent system may also include up to 15% by weight of the ink composition of a non-volatile high viscosity material having a boiling point greater than about 300 DEG C.

Description

The present invention pertains to the composition for an initially erasable ink for a ball point writing instrument which ink is characterized by its initial erasability by ordinary pencil erasers when applied by a ball point writing instrument to an absorbent paper-like writing surface and which thereafter develops permanence. If such a medium is to be considered, as being truly erasable, it must be readily removable at least during an initial period of time from the substratum to which it has been applied without any damage of any significant degree to the area of the substratum involved.
To understand the nature of this invention, requires at least an elementary knowledge of the structure of paper, since paper is the substratum most commonly employed as a writing surface.
Paper is essentially a mat of randomly oriented cellulose fibers. Thus, paper consists of solid structural members, namely the cellulose fibers, having numerous minute voids therebetween. From the description, it may readily be seen that the formulator or designer of an erasable writing medium must prevent the colorant portion of the medium from penetrating to any substantial degree into the voids in the surface being written upon because if there is substantial penetration into the voids the subsequent removal of the colorant by mechanical means cannot be accomplished without damage to the writing surface. Likewise, it may readily be seen that for a writing medium to be erasable, the colorant particles must be prevented from affixing themselves with any substantial degree of permanence, either by chemical reaction or as a result of mere physical attraction, to the solid members of the paper substratum.
In the past, attempts to achieve an erasable writing medium for a ball point pen usually have been unsuccessful. Of course, one may use a superabrasive eraser to remove ordinary ball pen inks from the paper substratum normally used for writing purposes.
Attempts have been made to modify ordinary ball pen ink so as to prevent the colorant portion thereof from penetrating the pores of the paper substratum. These attempts consisted of substituting pigment-type colorants for the dye-type colorants normally used in ball pen inks. The colorant portion of such inks was not immobilized to any significant degree and, as a consequence, no substantial advantage was gained through substituting the pigment-type colorant for the dye-type colorant in ordinary ball pen inks insofar as erasability was concerned.
Some success has been achieved in producing erasable ball point pen inks that are transitorily erasable, for example as disclosed by U.S. Patent No. 4,097,290 hereinafter referred to as Muller et al. and U.S. Patent No. 3,875,105 to Daugherty et al. Both of these patents teach the use of an elastomer. The Daugherty et al. patent teaches polyvinyl methyl ether and the parent application thereof, serial No. 751,759,1:eaches Natsyn 2200 which is a synthetic rubber having a chemical structure like natural rubber, while Muller et al. teaches natural rubber or rubber which essentially duplicates the chemical structure of natural rubber. Both patents disclose the use of a volatile solvent. At this point it should be noted that the term "solvent" is being used herein as a general term of art, and not in its specific technical sense, to describe the vehicle into which large elastomer molecules, which may tend to remain in small clusters, are distributed and wherein the vehicle serves to carry the elastomer molecules from within a ball pen cartridge onto the substrate.
Muller et al. discloses a volatile low boiling organic solvent for the elastomer having a boiling point less than 180°C and exhibiting 100% evaporation within 60 minutes in combination with a non-volatile organic liquid solvent having a high boiling point in excess of 300°C. In summary, both Muller et a/. and Daugherty et al. teach an erasable ink that includes the following: (1) the use of a pigment as a colorant; (2) an elastomer; and (3) the use of mixtures of volatile and non-volatile solvents.
It is the object of the present invention to produce an erasable ball point ink composition that possesses very smooth writing characteristics and that becomes permanent quicker, i.e. within hours, without compromising its short term erasability by ordinary pencil erasers.
The present invention provides an ink, for a ball point writing instrument, capable of depositing on an absorbent, paper-like writing surface having minute voids, an initially erasable but eventually permanent trace, and according to the invention the ink comprises the following components:-

a) an elastomer, being natural rubber or a synthetic rubber which essentially duplicates the chemical structure of natural rubber, or a mixture of natural rubber and such synthetic rubber, present in an amount of at least 18% by weight of the ink,
b) pigment, present in an amount of at least 10% by weight of the ink,
c) a solvent system comprising
- c¹) a volatile component having a boiling point less than 180°C and present in an amount of at least 8% by weight of the ink, and
- _C ²) a non-volatile component having a boiling point between 180°C and 300°C and present in an amount of at least 21% by weight of the ink.
d) conventional additives, as necessary, to make up 100% by weight.

Smoother writing erasable ball-pen inks of the present invention have been prepared that eventually become permanent quicker and more efficiently, without compromising short-term erasability with a pencil eraser, by using very low viscosity solvents (1.0-15 cps) such as light bodied oils that are further characterized by having boiling points or ranges between 180°C and 300°C at atmospheric pressure. Furthermore, the content of vehicles whose viscosities are above 15 cps and that boil above 300°C is minimized, and when used are employed in amount less than 15 percent by weight of the total ink composition.
The inventor made a study to define the role of these very low viscosity solvents in terms of comparing their effect on ink performance to oils and plasticizers that are significantly more viscous and that boil above 300°C. More specifically, erasable ball-pen inks (particularly those that depend on very volatile solvents to confer erasability using rubbery elastomers) undergo rapid changes in viscosity as the ink film leaves the cartridge via the ball socket onto the writing paper. The performance of higher viscosity oils and plasticizers, although useful for pigment dispersion as dispersants or plasticizers for rubber elastomers and as aids to achieving eventual ink permanence, show a direct relationship between their much higher viscosity and the perceived decrease in writing smoothness and the increased length of time required to achieve satisfactory permanence.
Shear stresses between the exposed ball and socket rim are efficiently reduced if the elastomer pigment matrix is flowing through the pen tip immersed in very low viscosity liquids (1-15 cps) rather than in relatively thicker vehicles (58-104 cps) such as dioctyl phthalate, diisodecyl phthalate or heavy mineral oils. It should be noted that the quantity of preferred volatile solvents (those boiling at about 94-120°C and evaporating in less than about 8 minutes) is rapidly diminishing as the ink film passes the socket rim onto the exposed surface of the ball.
In other words, a relatively non-volatile oil such as 2257 from Penreco, which has a viscosity about 39 times less than dioctyl phthalate, offers the rubbery pigment matrix less resistance to flow with the concurrent perception of increased writing smoothness. Through the mechanism of capillarity, the elastomer/pigment-oil matrix also incurs less resistance in penetrating surface paper fibers, again with the resulting decrease in the length of time required to achieve the desired degree of permanence. It should be noted that the rate of capillary absorption is inversely proportional to the viscosity.
Other advantages of using low viscosity essentially are non-volatile solvents besides those discussed above are: less stringing, resulting in a cleaner line; less tendency to transfer because of increased penetration of ink into paper; and less tendency of increased writing drag because low viscosity oils or solvents insure against the negative effects of elastomer molecular weights that are higher than the optimum desired values for satisfactory ink manufacturing.
The acceptable range of components for the present invention has been found to be as follows:
The preferred range of components, percent by weight of the ink, has been found to be as follows: elastomeric polymer 23-26%, colorant 18-20%, volatile solvent 19-21 %, non-volatile low viscosity solvent 26-31 %, non-volatile high viscosity solvent less than 5%, and lubricants 2-3%.
The elastomer is selected from the group consisting of natural rubbers, synthetic rubbers and mixtures thereof and preferably is a mixture of natural and synthetic rubbers in approximately equal amounts. It is preferable that the elastomeric material or materials chosen exhibit a predetermined substantially uniform molecular weight distribution over a wide molecular range of about 100,000 to 900,000. Insurance against material or process variability may be provided by using 50/50 mixtures of Natsyn 2205 and SMR 5CV60, synthetic and natural rubbers respectively.
Examples of suitable pigments include Victoria Blue, Alkali Blue, Phthalo Blue, Lithol Red, Red 2B, graphite, carbon black and Diarylide Yellow. The suppliers of such pigments are as follows:
Other colors will, of course, require the use of other pigments and there is no intent to limit the pigments to those listed. The only requirements of the pigments are that they import the desired color to the ink and that they are sufficiently fine to readily pass through the clearance between the ball and retaining lip of the ball point; i.e., approximately 5 microns or less.
A preferred low boiling point or volatile solvent is Lacquer Diluent #6 supplied by Chem Central Company of Chicago, Illinois which has the property of being compatible with polyisoprene and has a boiling point between 94°C-120°C. Suitable volatile solvents for the purpose of the present ink composition are considered to be those compatible with rubber having a boiling point less than 180°C at atmospheric pressure and evaporating in less than 60 minutes. Included among the suitable volatile solvents are V M & P Naphtha having a boiling point range of 121-139⁰C and 100% evaporation in 8.3 minutes, 360-66 Naphtha having a boiling point range of 154-173°C and 100% evaporation in 42.1 minutes, as well as very volatile solvents such as hexane and pentane.
Suitable non-volatile low viscosity solvents include liquids having a viscosity within the range of 1-15 cps and having a boiling point less than about 300°C and greater than about 180°C and particularly include light bodied oils such as hydrocarbon oils and light mineral oils boiling between 180°C and 300°C and having a viscosity around 1.5 centipoise which viscosity is sufficiently low as to result in a final ink viscosity of between 1,000,000 and 4,000,000 centipoise. A preferred example would be Penreco 2257 oil, a product of Penreco, headquartered in Butler, Pennsylvania, a division of Pennzoil Company of Houston, Texas. Penreco 2257 oil has a viscosity of 1.5 centipoise at 25°C and a boiling range of about 220°C to 260°C. However, the non-volatile low viscosity component may also be a petroleum derivative such as kerosene having the aforesaid low viscosity and low boiling point; a low viscosity, low boiling point essential oil such as pine oil, a terpene alcohol; or this component may be a low viscosity, low boiling point plasticizer such as dimethyl phthalate and Kodaflex (TXIB) marketed by Eastman Kodak Company, Kingsport, Tennessee. Table 3 below presents a listing of the physical properties of such suitable non-volatile low viscosity components, including 2257 oil.
A suitable non-volatile, high viscosity solvent or plasticizer may be chosen from any one of the compounds listed in Table 4 below, but typically include plasticizers and heavy mineral oils having boiling points in excess of 300°C.

Sources for materials & data

Mineral oils:

1. Penreco; Los Angeles, California.
2. Magie Bros. Oil Company; Los Angeles, California.

Plasticizers and solvents:

1. The Solvent and Chemical Companies; Los Angeles, California.
2. Eastman Kodak; Kingsport, Tennessee.

Suitable lubricants are those usually included in ballpoint pen ink formulations and include fatty acids such as oleic, stearic and lauric acids.
Where hydrocarbon resins are shown, a suitable hydrocarbon resin is Nevchem 140 manufactured by Neville Chemical Company, Pittsburgh, Pennsylvania.
The following examples are illustrative of preferred embodiments but should not be construed in any way as limiting the present invention. In these examples components designated as volatile solvents have a boiling point less than 180°C at atmospheric pressure and the non-volatile low viscosity solvents have a viscosity of 1-15 cps and a boiling point ranging from 180°C to 300°C. The non-volatile substantially high viscosity solvent or plasticizer has a viscosity of 58 cps or more and a boiling point greater than 300°C at atmospheric pressure.
The ballpoint pen inks of this invention are more viscous than the customary glycol based ballpoint pen inks and therefore are more suitable for use in a pressurized cartridge. The exact pressure required will be dependent upon the opening between the ball and lip of the point used, the viscosity range of the final ink composition, and the amount of ink and volume of the tube used to contain the ink. It has been found that one desirable ink pressure medium is compressed air.
The pressurized cartridge utilizes a solid piston that is positioned on top of the ink supply and forwardly of the pressure producing medium. The follower has a central section that is cylindrical in shape and has its two ends formed as conical portions so that no specific orientation in assembly is required.
The following names used in the above description are believed to be Registered or other proprietary Trade Marks.

Penreco
Natsyn
Diarylide
Kodaflex
Magie
Shell Evaporater
Nevchem

Claims

1. An ink, for a ball point writing instrument, capable of depositing on an absorbent, paper-like writing surface having minute voids, an initially erasable but eventually permanent trace, the ink comprising the following components:-

a) an elastomer, being natural rubber or a synthetic rubber which essentially duplicates the chemical structure of natural rubber, or a mixture of natural rubber and such synthetic rubber, present in an amount of at least 18% by weight of the ink,

b) pigment, present in an amount of at least 10% by weight of the ink,

c) a solvent system comprising

_C ¹) a volatile component having a boiling point less than 180°C and present in an amount of at least 8% by weight of the ink, and

c²) a non-volatile component having a boiling point between 180°C and 300°C and present in an amount of at least 21% by weight of the ink.

d) conventional additives, as necessary, to make up 100% by weight.

2. An ink according to claim 1 wherein the elastomer component a) is present in an amount of 18 to 28% by weight of the ink.

3. An ink according to claim 1 or 2, wherein the elastomer component a) has a substantially uniform molecular weight distribution over the range of 100,000 to 900,000.

4. An ink according to any foregoing claim wherein the elastomer component a) is a mixture of approximately equal amounts of natural rubber and synthetic rubber.

5. An ink according to any foregoing claim wherein the pigment component b) is present in an amount of 10 to 22% by weight of the ink.

6. An ink according to any foregoing claim wherein the volatile component c¹) is present in an amount of 8 to 30% by weight of the ink.

7. An ink according to any foregoing claim wherein the non-volatile component c²) is a hydrocarbon oil having a viscosity in the range of 1 to 15 cps and present in an amount of 21 to 50.5% by weight of the ink.

8. An ink according to any foregoing claim wherein the solvent system c) additionally comprises

c³) a high viscosity material having a viscosity of at least 58 cps and a boiling point greater than 300°C and present in an amount of up to 15% by weight of the ink.